Epstein-Barr virus (EBV) is a member of the herpes virus family and is present in all human populations. Primary infection usually occurs in early childhood and remains silent throughout a person's life. However, when uninfected adolescents and young adults are exposed to EBV, about 60% manifest infectious mononucleosis (IM).
The predominant laboratory test used to establish the diagnosis of IM has been the demonstration of heterophil antibodies. The rapid slide tests have become the most widely used method to detect these heterophil antibodies. In contrast, quantitative agglutination tests, such as the Paul-Bunnell-Davidsohn method, are more accurate but are also more tedious and time consuming.
The use of tests to measure heterophil antibodies have limitations, namely, only between 80-95% of IM patients produce these antibodies, these antibodies are absent in a large percentage of young children and the antibodies are produced in a variety of other diseases such as lymphoma, hepatitis and leukemia. The measurement of heterophil antibodies also does not give any indication of the severity of the disease and cannot be used to monitor the course of IM.
Immunofluoresence tests that measure antibodies to EBV can also be used in the diagnosis of heterophil-negative cases of IM, or patients with atypical manifestations.
These tests, however, are time consuming and require the use of trained personnel and specialized equipment which does not make them amenable to the routine analysis of large numbers of samples.
The diagnosis of an acute primary EBV infection can also be determined by an IgM response to EBV-viral capsid antigen (VCA). The VCA is composed of a large number of different antigens. Components of VCA are defined by the fact that they are expressed late in the replicative cycle of the virus. Many VCA components have been mapped to specific open reading frames (ORF's) within the EBV genome though there are many ORF's, known to be expressed late in replication, to which specific VCA antigens have not yet been identified.
Genetic engineering and synthetic polypeptide technologies now enable the manufacture of large quantities of protein and polypeptide antigens. However, these techniques are only effective if the amino acid residue sequence of the native protein is known.
The amino acid residue sequence of a natural protein can be determined by sequencing of the protein itself.
Alternatively, the DNA sequence that codes for the protein may also reveal the protein's amino acid residue sequence.
Antibodies can be used to determine whether an ORF present in a DNA sequence codes for a protein. This involves manufacturing an array of protein fragments or synthetic polypeptides whose amino acid residue sequences correspond to the hypothetical sequences obtained from the ORFs. The protein fragments or polypeptides to which naturally occurring antibodies immunoreact thereby identify the ORF as encoding a naturally occurring protein. The complete amino acid sequence of this protein could then be deduced from the DNA sequence of the ORF.